Yes, the Send() method blocks until the delegate completes. It can be used to run code on the main thread, blocking the current thread (unless the two are the same). An example I took inspiration from is Unity's reference implementation.

The role of OperationStarted() and OperationCompleted() is supporting async void methods according to this article. These methods don't return a Task object so there's no way for the caller to keep track of the task's state or any exceptions that might occur. The OperationStarted() method gets called when the method begins executing and the OperationCompleted() method is called when the method is finished. Exceptions are posted to the context as well.